1. Field of the Invention
This invention relates to a hydraulic braking pressure control apparatus for vehicles, and more particularly to an apparatus having an input hydraulic chamber communicating with an output port of a master cylinder, and an output hydraulic chamber communicating with a wheel brake and being adapted to generate a hydraulic braking pressure in accordance with a hydraulic pressure in the input hydraulic chamber, wherein the volume of the output hydraulic chamber can be increased in accordance with the supply of a hydraulic control pressure from an anti-lock control means to a control chamber when a wheel is about to be locked.
2. Description of the Prior Art
In a conventional hydraulic braking pressure control apparatus for vehicles, a piston is operated and moved in response to the introduction of a hydraulic pressure into an input hydraulic chamber so as to reduce the volume of an output hydraulic chamber and thereby generate a hydraulic braking pressure from the output hydraulic chamber in accordance with the pressure in the input hydraulic chamber. During an anti-lock control operation, the piston is displaced in a direction opposite to that in the abovementioned case by means of a control liquid pressure supplied to a control chamber, to increase the volume of the output hydraulic chamber.
In the above conventional hydraulic braking pressure control apparatus, a hydraulic braking system is divided into two parts, one of them extending from a master cylinder to an input hydraulic chamber, and the other extending from an output hydraulic chamber to a wheel brake. Accordingly, when supplying a working oil to the hydraulic braking system, it is required that those two parts be charged with the oil separately. Moreover, since the piston is in operation at all times during a braking operation, the number of its operation strokes increases to a high level. This may lead to deterioration of the durability of the control apparatus.
The assignee of the present invention has already proposed a hydraulic control apparatus provided with a valve mechanism of a normally opening type in a partition located between input and output hydraulic chambers, which valve mechanism is adapted to disconnect the input and output hydraulic chambers from each other during an anti-lock control operation, in order to integrate the hydraulic braking system into a single line extending from a master cylinder to a wheel brake, thereby facilitating a working oil charging operation and reducing the number of strokes of a piston to improve the durability of the control apparatus
According to such a hydraulic control apparatus, the above-mentioned problems can be solved. However, when the vehicle runs on a bad road, or when the vehicle is braked excessively due to the trouble of an anti-lock control means, the abnormal increase of the hydraulic pressure in a control chamber causes the volume of the input hydraulic chamber to decrease abnormally and that of the output hydraulic chamber to increase abnormally. For this reason, the working oil in the input hydraulic chamber may be returned to the master cylinder more than necessary, and kicking-back may occur in a brake pedal more than necessary. In addition, a negative pressure may be developed in the hydraulic system between the output hydraulic chamber and wheel brake to cause the generation of trapped air bubbles.